1. Field
The present invention relates generally to object detection, and more particularly to systems for sensing a composition and/or a location of an object.
2. Description
Non-contact sensors may be used to determine a distance between two objects. Such sensors are sometimes useful for anticipating and/or avoiding collisions. In one approach, a non-contact sensor is mounted on a first object and determines a distance between the first object and a second object. If the second object is less than a predetermined distance from the first object, additional system elements may output a warning or execute an avoidance maneuver.
Non-contact sensors may operate based on inductance or capacitance. Inductive sensors may be particularly suitable for detecting metallic objects, while capacitive sensors may be more suitable for other types of objects, including biological materials. A typical capacitive sensing system generates an electric field between a sensor and an object of interest and measures a capacitance between the sensor and the object based on a magnitude of the electric field. The sensor then determines a distance to the object based on the measured capacitance. A significant portion of the electric field flows from the sensor to ground rather than to the object, thereby decreasing the accuracy of the determined distance.
U.S. Pat. No. 5,166,679, entitled “Driven Shielding Capacitive Proximity Sensor”, describes a particular type of capacitive sensor known as a “capaciflector”. The capaciflector attempts to provide greater sensitivity than traditional capacitive sensors by reducing a portion of the generated electric field that flows between a sensor element and ground.
FIG. 1 illustrates the operation of a capaciflector according to the above-mentioned patent. As shown, capaciflector 10 is positioned to sense object 20. Capaciflector 10 includes sensor element 12, dielectric 14, shield element 16 and dielectric 18. Sensor element 12 and shield element 16 may be composed of dissimilar materials, and dielectrics 14 and 18 may also be composed of dissimilar materials.
Capaciflector 10 is mounted to grounded structure 30. The elements of capaciflector 10 are not necessarily drawn to scale, and may comprise layers of extremely small thickness in comparison to dimensions of structure 30. In this regard, a distance between capaciflector 10 and object 20 may be substantially equal to a distance between structure 30 and object 20.
During some examples of operation, sensor element 12 and shield element 16 are both electrically coupled to an input signal. Accordingly, substantially no electric field is generated between sensor element 12 and shield element 16. Electric field lines therefore emanate primarily from sensor element 12 toward object 20, with only some, if any, field lines flowing from sensor element 12 to structure 30. The resulting range and sensitivity of capaciflector 10 may be substantially greater than that of other capacitive sensors.
As described above, the distance between a capacitive sensor and an object is determined based on the capacitance therebetween. However, for a given distance and input signal, the capacitance may vary based on a material of which the object is composed. The accuracy of current proximity sensors therefore depends on the material of the object to be sensed.
In view of the foregoing, a substantially material-independent system is desired for accurately and efficiently determining a distance to an object.